Reflow soldering is a commonly used process for connecting components to a circuit board. The components are placed on solder paste located on the circuit board and the board is heated in an oven to melt the solder and thereby connect the components.
The ovens used are arranged to provide some control over the temperature throughout the process. Producing the correct temperature profile for the process is extremely important to producing good connections between the components and the board. A common process uses an initial preheat zone in which the temperature is ramped up, a thermal soak zone in which temperature is maintained relatively constant and a reflow zone in which the temperature is ramped up again to melt the solder. The rate of increase of temperature in the preheat and reflow zones is particularly important in obtaining good connections. A final cooling zone is also used to cool down the board and components. Again, the rate of cooling is important to the connection quality.
A number of different ovens for this purpose are available which use different methods of heating the board. Convection and infrared radiation are two such methods which have been employed either separately or in combination. Ovens using these heating techniques may utilise a method of moving the board through various zones in the oven in order to provide the different temperatures required at the various stages of the process.
A further method involves the use of vapour phase heating. In such an oven, a liquid with a predefined boiling point is heated until a vapour of the liquid is formed. The liquid used is one such that the vapour produced is heavier than air and therefore forms a vapour layer which sits above the liquid in a vessel. The board is then lowered into the vapour layer to apply heating. In order to control the temperature of the board throughout the process, a means of moving the board in and out of the formed vapour layer may be employed.
Vapour phase ovens are known to have poor control of the temperature profile causing problems such as thermal shock to parts and increased tombstoning (being parts standing up during the reflow process) compared to convection ovens.
Infrared and convection ovens result in higher peak temperatures and variations in peak temperatures and temperature profiles for parts across the PCBs and this leads to higher failure rates.
The present invention relates to an improved method and apparatus for applying heat to a circuit board for the purpose of reflow soldering.